ANATOMY AND PHYSIOLOGY 



97 



C -T. 



extremely limited; for since the form of the lens is fixed and also the 

 distance between the lens and the retina, there is no power of accom- 

 modation, and most external objects are out of focus; to -make an 

 image, then, the object must be at one definite distance from the lens, 

 and as the lens is usually strongly convex, this distance must be small; 

 in other words, insects, like spiders, are very near-sighted, so far as the 

 ocelli are concerned; furthermore, the small number of retinal rods 

 implies an image of only the coarsest kind. 



If the compound eyes of a grasshopper are covered with an opaque 

 varnish and the insect is placed in a box with only a single opening, it 

 readily finds its way out by means of its ocelli; if the three ocelli also 

 are covered, however, it no longer does so, 

 except by accident, though it can make its 

 escape when only one of the ocelli is left 

 uncovered. The ocelli, then, can distinguish 

 light from darkness and they are probably 

 more serviceable to the insect in this way 

 than in forming images. 



Compound Eyes. As regards delicacy and 

 intricacy of structure, the compound eye of 

 an insect is scarcely if at all inferior to the eye 

 of a vertebrate. In radial section (Fig. 143), 

 a compound eye appears as an aggregation 

 of] similar elongate elements, or ommatidia, 

 each of which ends externally ina facet. The 



following Structures Compose, Or are Concerned vomitoria, radial section, 



.,, , ,. ,. / \ / \ nea; i, iris pigment; n, nerve 



with, each ommatidmm: (i) cornea, (2) crys- fibers . nCt nerve cells; r. retinal 

 talline lens, or cone. (3) rhabdom and retinula, pigment; *, trachea. After 



HlCKSON. 



(4) pigment (iris and retinal), (5) fenestrate 

 membrane, (6) fibers of the optic nerve, (7) trachea. 



The cornea (Fig. 144) is a biconvex transparent portion of the exter- 

 nal chitinous cuticula. Immediately beneath it are the cone cells, which 

 may contain a clear fluid or else, as in most insects, solid transparent 

 cones. The rhabdom is a transparent chitinous rod or a group of rods 

 (rhabdomeres) situated in the long axis of the ommatidium and sur- 

 rounded by greatly elongated cells, which constitute the retinula. 

 Two zones of pigment are present: an outer zone, of iris pigment, in 

 which the pigment in the form of fine black granules is contained 

 chiefly in short cells that surround the retinula dis tally; and an inner 

 zone, of retinal pigment, in which the pigment cells are long and slender, 



